PT design Ieff=Ig

[rowingengineer]

following from thread "Post Tensioning as a Deferred Submittal" a discussion about Ieff=Ig for PT has been sparked, trying not to take over this thread I have started a new thread.

Lion06 had mention that Ieff=Ig is allowed by the ACI code, i believe this is in reference to ultimate design, and not serviceable design as Asixth was describing.

In most flat slab PT designs in Australia the design is for PT bonded combined with reo, thus it is likely that the slab is going to crack under loading at some stage. so a Ieff=Ig assumption is not correct and needs to be reviewed for each loading case. However I limit my maximum Ief to 0.7Ig.

The reason we do this is for restrained shrinkage and other similar effects.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[rowingengineer]

Lion06,
There are a number of different elements to restraint design, these are based on a number of different aspects and has not as yet been fully tested. I doubt even if you program was using a shrinkage design value in the loading that the results would be any more correct. truth is that restraint shrinkage is occurring while creep, reo restraint are progressing.

This makes it very hard to define and give an appropriate number during analysis. The designer needs to be aware of the situation and include a appropriate adjustment to account for this within his design. This can be conducted in many different manners.

I once wrote an article that I hoped to have published, but alas that never happened. I have posted a copy of this article under another post as I will be seeking discussion of this article for all perspectives. I hope you will take the time to review this article, it discusses FE modeling and associated interesting aspects. I also hope you will provide comment on the article.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[Lion06]

I have not gone through the calcs by hand. I do wish I had the time, but, to be honest, I'm backchecking one software with another.

As for the program taking shrinkage and creep into account........ I'm not saying the model does that. The point I was trying to make is what's the difference whether I let the program do it and I use the actual jacking force for the cables or if I neglect the creep and shrinkage in the model, but use a final effect force that is reduced for creep and shrinkage? I'm not being a smartass, I'm honestly asking the question, because I'm not seeing the difference. Why is this any different than the friction and seating losses. You don't model the friction of the tendons or the actual wedges getting seated, but it still works out ok, presumably because you account for this with the prestress force used. What's the difference?

As for the FE analysis accounting for the restraint from walls - how is this not accounted for with the analysis. The walls (with their correct stiffness) is modeled and connected to the slab as it will be constructed. The walls are preventing the slab from shrinking. I believe this to be true, because I've done some trial runs of identical strips with and without walls at the ends and there is a difference in P/A, so that leads me to the conclusion that the analysis is capturing the restraint offered by the walls.

 

[rapt]

Lion06,

1 How do you know which one is correct, or if either is correct?

2 We are talkin about 2 different things.
- You are talking about the effects of shrinkage and creep on the prestressing force. Yes, creep and shrinkage reduce the prestress force and traditionally this is calculated before the analysis and included as an effective prestress force. WE are not disagreeing with this.
- In determining the stresses in a concrete member, vertical loads and prestress will cause stresses that we account for. Most designers ignor other stress in the member that are less obvious and harder to calculate. The biggest of these is restraint to shortening from shrinkage and temperature change. These are the stresses we are saying need to be included in the calculations if you want to use Fct as the tensile strength of the concrete and Ieff = Ig in deflection calculations.

3 It is not accounted for in the analysis because the shrinkage is not modelled as an effect that produces stress and strains in the concrete in RAM Concept and other design programs.
I told you that they allowed for the shortening effect on the prestress force. And that is what you are seeing. Its effect has been modelled in a simplistic way that is giving a result which is not really correct, but it has been modelled. Construction sequence needs to be included in the modelling to get more realistic answers but cannot be done currently.
But you are not seeing any stresses induced in the concrete by restraint to shrinkage and temperature change, because neither effect has been modelled in the program.

You cannot look at results from computer programs whose internal workings you have no idea about or understanding of and make the assumptions that you are. That is not engineering. You should not even be using software if you have no design experience in the area. Complicated software is for use by experienced designers. And you are not one in this area by your own admission!

 

[hokie66]

I tend to agree with rapt. We should all stop using software written by someone else, as none of us understands the internal workings.

In other words, back off, rapt. Lion06 is not one of the irresponsible ones.

 

[rapt]

Hokie66,

You raise a philosophical arguement that I have been debating with myself (worrysome) and others for some time. Is design software the cause of declining engineering standards? Is software a black box because
- it is written by someone other than the user,
- or is it because the user has not investigated what it does properly and so does not understand it
- or is it because the user has no experience in the field and expects the software to be 100% correct no matter what input is supplied and to replace his lack of understanding of the design area?

I suggested in one of my responses that RAPT is a black box and received a private reply below
"also I disagree with rapt being a black box or grey, your help manual, (while the manual is slowly getting out of date) has the theory for what a program is doing this is better than most programs. I don't use black box solutions."

When you start thinking about it, if software is a black box, text books and design codes are also. Incorrect interpretation of or illogical data into those will also suffer from the garbage in - garbage out problem. I have had a lot of experience with people doing this in design codes when they have questioned RAPT's results compared to their hand calculation from code formulae. Some of the interpretations of code rules and manipulation of data to get the desired result is amazing.

So should everyone go back to developing their own design theories because text books and codes are open to interpretation?

After all of my debate on this (with myself and others), the conclusion is that
- engineers are responsible for understanding the design area they are involved in. They should not use design tools of any sort until they understand the theory and practice in that area, and then they should not use the tools productively until they understand the tool. The theoretical education of the designer/user is not the responsibility of the software company.

- Software developers should explain what their software does and put in the effort to try to make the software as useful to the designer as possible in applying the rules logically and correctly, and market it ethically not in a way purely to boost sales!

And all of this gets down to education of designers, both at university and in practice. To me, that is Lion06's problem as he is not being given time to to what he should do (by his own admission). Is that my fault! Is it his bosses fault? Is it the industry's fault?

This forum can help people with questions on design and interpretation of design logic. It cannot teach designers a complete area of design.

 

[hokie66]

rapt,
I have debated the same philosophical questions myself over the years. The first programs I used were self written, stored either on punch cards for running on an IBM360 (while in university), or on paper tape for a little HP box. As analysis software became available commercially, then design programs, then all sorts of combinations like we have today, it has become more and more difficult to interrogate and verify software. Thus, it has for many of us become a matter of developing confidence, and that is what Lion06 is trying to do.

Your contributions to the site are valued by many here, but in reality, and with no disrespect, you are somewhat of a "one trick pony" when compared to many of the practicing engineers trying to deal with a wide variety of materials, methods, code requirements, and structural forms. My snarky comment was just because I thought you were tending toward being disdainful of genuine questions, and that is not going to help anyone.

 

[rowingengineer]

There is an obvious problem with the computer programs becoming too complex, the reason for engineers not spending the time to review these to an in-depth level is due to many factors, costs, time, experience, understanding, and programs documentation.

What we do need to encourage is for engineers to spend the time reviewing the inner workings of a particular software. weather it be a finite element program like microstran for steel design or concrete design. each time these turn up at our desk we should ask those around use what errors they have found and to what depth they have reviewed. We should also be keen to do some calcs by hand to review the effectiveness of the software, but in saying this I don't think you need to design a complete building by hand and then compare every aspect. I think you need to review it in pieces, choosing simple concepts to review, ie torsion or high shear ect. once this is completed I think your understanding of the program will ensure that you don't allow garbage out.

But getting back on topic,

in regards to lions question "As for the FE analysis accounting for the restraint from walls - how is this not accounted for with the analysis. The walls (with their correct stiffness) is modeled and connected to the slab as it will be constructed. The walls are preventing the slab from shrinking."

we need to establish the programs basis, I don't know ram concept for PT design as well as I would like to for the purposes of this conversation. does the program allow for a shrinkage theory in the manual? if so how does this theory work? what type of model does it use? plate or shell.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992